1. Field of the Invention
The present invention relates to a rotation information detection apparatus, and more particularly to an apparatus which is well applicable to a rotary encoder for measuring movement information of a diffraction grating, that is, movement information of a moving object by irradiating a coherent light beam such as a laser beam to fine grating arrays such as a diffraction grating of a disk mounted on the moving object (scale), causing diffracted lights of a predetermined order from the diffraction grating to interfere each other to form an interference fringe, and counting numbers of light and dark fringes of the interference fringe.
2. Related Background Art
A rotary encoder has been known as an instrument to measure rotation information such as a rotation amount or a direction of rotation of a rotating object in an NC machine tool and the like with a high precision by using a unit of sub-micron and the like, and it has been widely used.
As a high precision and high resolution rotary encoder, a diffracted light interference type rotary encoder which determines a movement state such as a a movement distance of the moving object or other movement information by irradiating a coherent light beam such as a laser beam to a diffraction grating mounted on a moving object, causing diffracted lights of a predetermined order from the diffraction grating to interfere each other and counting numbers of light and dark fringes of the interference fringe, has been well known.
In this type of diffraction grating type rotary encoder, when high resolution and high precision are to be attained by using a fine grating (radial grating), only the diffracted lights of a predetermined order of the diffracted lights created by the fine grating are extracted by an optical system and they are superimposed in the same optical path by an appropriate optical means to get an interference signal.
In general, in a rotary encoder, it has been required that:
(1-a) A disk (rotary disk) of a small diameter having radial gratings recorded thereon with a high density is used and it is of high resolution and low inertia. PA1 (1-b) An entire apparatus is thin and compact. PA1 (1-c) The apparatus may be directly mounted as a unit on an object under test by separate a disk and a detection head therefrom, and it is easy to assemble. PA1 (1-d) A measurement precision is stable against a change in an environment, particularly a temperature. PA1 (1-e) Deterioration of an output signal and reduction of the measurement precision due to an assembly error of the disk and the detection head does not arise therein.